1. Field of the Invention
The present invention is directed to a nuclear magnetic resonance tomography apparatus of the type having a basic field magnet with shim coils built into the basic field magnet, the shim coils being driven with shim currents such that a uniform magnetic field is produced in a slice of an examination volume the slice being smaller than the examination volume.
2. Description of the Prior Art
The homogeneity of the basic magnetic field is a critical factor for the quality of the image in nuclear magnetic resonance tomography apparatuses. Deviations from ideal homogeneity lead, for example, to spatial distortions.
Pulse sequences for the excitation and read-out of nuclear spins are sensitive to magnetic field inhomogeneities to different degrees. For example, relatively large magnetic field inhomogeneities can be tolerated with pure spin echo methods. In methods that are based on gradient echoes, by contrast, higher demands are made on the homogeneity. Especially high demands are made, for example, in echo planar imaging (EPI), wherein a number of nuclear magnetic resonance signals are acquired in a short time by multiple switching of a gradient after the excitation, or in turbo-gradient spin echo imaging wherein an excitation is followed by a number of 180.degree. pulses between which a read-out gradient with alternating polarity is activated. Extremely high demands are made of the basic field homogeneity in the examination region, moreover, in functional imaging and localized spectroscopy in order to be able to adequately resolve the spectral shift.
Whereas, in the case of low demands on the magnetic field homogeneity, it suffices to shim the basic field magnet once, shimming dependent on the respective examination subject must ensue given higher demands. For example, U.S. Pat. No. 5,345,178 discloses a suitable method. Nuclear magnetic resonance signals are thereby excited and read out before each image data measurement in the examination region. Specific shim coils are provided for the shimming, with the shim current for these shim coils being calculated by evaluating the nuclear magnetic resonance signals.
In general, the homogeneity can be made better for a specific region the smaller this region is. For example, the half intensity width of a magnetic resonance line can serve as a criterion for the homogeneity. The magnetic resonance line becomes narrower the more uniform the magnetic field is.
Usually, the entire nuclear magnetic resonance signal that is received by the antenna of a nuclear magnetic resonance apparatus after a non-localized excitation is used for setting the homogeneity. Given head applications with a specific head antenna, a line width of the magnetic resonance signal of about 15 through 25 Hz is achieved with this "global" shimming. The homogeneity can, however, still vary significantly within the head.
U.S. Pat. No. 4,700,136 discloses that the homogeneity can be locally improved even further when only the signal from the region (slice or volume part) wherein the actual measurement is to occur later is used for setting the homogeneity. Thus, the magnetic field is only locally shimmed. When, for example, a volume element in the head having a size of 10.times.10.times.1 cm is shimmed, then line widths of about 8 Hz are achieved. This known method, however, is limited to one volume element, or one slice.